[FR] Add support for placeholder width and quality in config

[rlapierre-kffein]

It seems that placeholder's width and quality is not updatable in Placeholder service (default values are 16px and 50% quality as constant).
It would be more flexible to have those values updatable in configuration file to get control over placeholder image quality.
There's already a configuration to enable this one 'generatePlaceholders' => true
Configuration can be something like placeholders => ['width' => 16, 'quality' => 50]